Water activity

WATER ACTIVITY
BY
ASSIST. PROF. Dr. BERCIYAL GOLDA. P

CONTENTS
Introduction
Definitions
Definitions
Concept of Water Activity
Why Water Activity is important?
Methods for measuring water activity
Factors affecting Water Activity
Microbial Growth of aw
Minimum aw for some bacterial growth
Examples of aw values of several foods
Advantages & Disadvantages of aw

•Most foods will not support the growth of bacteria if their water activity is less than 0.85, because at this water
activitythereisnot enoughwateravailableforthebacteriato grow.
•However, yeasts can grow at water activities as low as 0.70, while some molds will grow even at water activities
aslowas 0.60!
•Foods with water activities in this range usually have preservatives added to prevent the growth of yeasts and
molds.
•Acidic foods with a pH less than 4.6, such as tomato sauce, retard the growth of microorganisms. Thus an
acidic food with a water activity less than 0.85 is relatively shelf stable, especially if it is stored in the
refrigerator.
•In this case, low pH, water activity and temperature combine to provide good insurance against the growth of
harmfulpathogens.
Introduction

Definitions
Food scientists measure the amount of water that is available for the growth of microorganisms, as well as
enzymeandchemicalreactions,througha numberknownas wateractivity(aW).
Water activity is a measure of the amount of free and adsorbed water in food, and is measured on a
dimensionlessscaleof zeroto one.
Water activity is the ratio of the vapor pressure (P) of water in food divided by the vapor pressure of pure
water(P0)at thesametemperature.
Thewateractivityof purewaterisequalto 1.0.
Another way of determining water activity is by measuring the relative humidity (RH) of the atmosphere in
equilibriumwiththefood:RH (%)= 100 x aW.

Food Moisture% Water Activity
Fresh meat 70 0.98
Bread 40 0.95
Flour 14 0.70
Pasta 10 0.45
Potato chips 2 0.10
Inotherwords,wateractivityisa measureof thewaterthatis availableto be convertedto vapor.
Thereis a generalcorrelationof themoisturecontentof foodwithitswateractivity, asshowninthe table
below.

Conceptof WATERACTIVITY
The concept of aw has been very useful in food preservation
and on that basis many processes could be successfully adapted
and new products designed.
Water has been called the universal solvent as it is a
requirement for growth, metabolism, and support of many
chemical reactions occurring in food products.
Free water in fruit or vegetables is the water available for
chemical reactions, to support microbial growth, and to act as a
transporting medium for compounds.
In the bound state, water is not available to participate in
these reactions as it is bound by water soluble compounds such
as sugar, salt, gums, etc. (osmotic binding), and by the surface
effect of the substrate (matrix binding).
These water-binding effects reduce the vapour pressure of the
food substrate according to Raoult’s Law.

Comparing this vapour pressure with that of pure water (at the
same temperature) results in a ratio called water activity (aw).
Pure water has an aw of 1, one molal solution of sugar - 0.98,
and one molal solution of sodium chloride - 0.9669.
 A saturated solution of sodium chloride has a water activity of
0.755. This same NaCl solution in a closed container will develop
an equilibrium relative humidity (ERH) in a head space of 75.5%.
A relationship therefore exists between ERH and aw since both
are based on vapour pressure.
The ERH of a food product is defined as the relative humidity of
the air surrounding the food at which the product neither gains nor
loses its natural moisture and is in equilibrium with the
environment.
aw=ERH
100

Why is water activity important?
Water activity is a critical factor that determines shelf life.
While temperature, pH and several other factors can influence if
and how fast organisms will grow in a product, water activity may
be the most important factor in controlling spoilage.
Most bacteria, for example, do not grow at water activities
below 0.91, and most molds cease to grow at water activities
below 0.80.
By measuring water activity, it is possible to predict which
microorganisms will and will not be potential sources of spoilage.

Water activity--not water content--determines the lower limit
of available water for microbial growth.
In addition to influencing microbial spoilage, water activity can
play a significant role in determining the activity of enzymes and
vitamins in foods and can have a major impact their color, taste,
and aroma.
It can also significantly impact the potency and consistency of
pharmaceuticals.

METHODSFORMEASURINGWATERACTIVITY
Methods are based on the colligative properties of solutions.
Water activity can be estimated by measuring the following:
1. Vapour pressure
2. Osmotic pressure
3. Freezing point depression of a liquid
4. Equilibrium relative humidity of a liquid or solid
5. Boiling point elevation
6. Dew point and wet bulb depression
7. Suction potential, or by using the isopiestic method
8. Bithermal equilibrium
9. Electric hygrometers
10. Hair hygrometers

Water activity is expressed as the ratio of the partial pressure of
water in a food to the vapour pressure of pure water with the
same temperature as the food.
Thus, measuring the vapour pressure of water in a food system is
the most direct measure of aw.
The food sample measured is allowed to equilibrate, and
measurement is taken by using a manometer or transducer device
(Vapour pressure manometer).
This method can be affected by sample size, equilibration time,
temperature, and volume.
This method is not suitable for biological materials with active
respiration or materials containing large amounts of Volatiles.
1. Vapourpressure

2. Osmotic pressure
Water activity can be related to the osmotic pressure (p) of a solution
with the following equation:
p = RT/Vw ln(aw)
where Vw is the molar volume of water in solution, R the universal gas
constant, and T the absolute temperature.
 Osmotic pressure is defined as the mechanical pressure needed to prevent
a net flow of solvent across a semi-permeable membrane. For an ideal
solution, Equation can be redefined as:
p = RT/Vw ln(Xw)
where Xw is the molar fraction of water in the solution. For non-ideal
solutions, the osmotic pressure expression can be rewritten as:
p = RTfnmb(mwVw)
where n is the number of moles of ions formed from one mole of
electrolyte, mw and mb are the molar concentrations of water and the
solute, respectively, and f the osmotic coefficient, defined as:
f = -mw ln(aw)/nmb

3. Freezing point depression of a liquid
This method is accurate for liquids in the high water activity range but is
not suitable for solid foods (Barbosa-Cánovas and Vega-Mercado, 1996).
The water activity can be estimated using the following two expressions:
Freezing point depression:
-log aw = 0.004207 DTf + 2.1 E-6 DT2
f (1)
where DTf is the depression in the freezing temperature of water
Boiling point elevation:
-log aw = 0.01526 DTb - 4.862 E-5 DT2
b (2)
where DTb is the elevation in the boiling temperature of water.

4. Equilibriumrelativehumidityof a liquidor solid
The relative humidity is the water vapor pressure (numerator) divided
by the equilibrium vapor pressure (denomator) times 100%. The
equilibrium vapor pressure occurs when there is an equal
(thus the word equilibrium) flow of water molecules
arriving and leaving the condensed phase (the liquid or ice).
If the water vapor pressure is greater than the equilibrium value
(numerator is greater), there is a net condensation (and a cloud could
form, say).
And that is not because the air cannot hold the water, but merely
because there is a greater flow into the condensed phase than out of it.
Relative humidity describes the amount of water vapor actually in the
air (numerator), relative to the maximum amount of water the air can
possibly hold for a given temperature (denominator).

One important consequence is that when air masses change in temperature,
the relative humidity can change, even if the actual amount of water vapor
in the air does not (the numerator in our equation, which is defined by the
saturation curve, stays the same, but the denominator changes with
temperature).
If the relative humidity (RH) is 100%, this means that condensation
would occur. On a typical hot muggy summer day, RH might be around
60-80%. In a desert, RH is commonly around 15-25%.
It is expressed as a percentage:
RH=H2OactualH2OmaxThis equation is not rendering properly due to an
incompatible browser. See Technical Requirements in the Orientation for a
list of compatible browsers.

When an air mass contains the maximum amount of water it can hold, it is
saturated with water vapor. This is shown graphically in the plot above as
the black solid curved line in Figure .With increasing temperature (x-axis),
the air can hold more water vapor (y-axis), as indicated by higher
saturation values (solid black curved line).
In general, it is not possible to have water contents that exceed
saturation (i.e. relative humidity is 100%). In other words, the maximum
relative humidity is generally not greater than 100% (i.e. not above the
solid black curved line). Another way to think about relative humidity is
that it describes how close the air is to saturation.
In the example shown, the actual water vapor content is about 40% of
that at saturation (i.e. the blue point is about 40% of the way to
saturation) – meaning the RH = 40%.

5. Boilingpointelevation
oBoiling-point elevation describes the phenomenon that the boiling point of
a liquid will be higher when another compound is added, meaning that a
solution has a higher boiling point than a pure solvent.
oThis happens whenever a non-volatile solute, such as a salt, is added to
a pure solvent, such as water.
oThe boiling point elevation is a colligative property, which means that it
is dependent on the presence of dissolved particles and their number,
but not their identity.
oIt is an effect of the dilution of the solvent in the presence of a
solute.
oIt is a phenomenon that happens for all solutes in all solutions, even in
ideal solutions, and does not depend on any specific solute–solvent
interactions.
oThe boiling point elevation happens both when the solute is
an electrolyte, such as various salts, and a nonelectrolyte.

In thermodynamic terms, the origin of the boiling point elevation
is entropic and can be explained in terms of the vapor
pressure or chemical potential of the solvent.
In both cases, the explanation depends on the fact that many solutes are
only present in the liquid phase and do not enter into the gas phase
(except at extremely high temperatures).
Put in vapor pressure terms, a liquid boils at the temperature when its
vapor pressure equals the surrounding pressure.
For the solvent, the presence of the solute decreases its vapor pressure
by dilution.
A nonvolatile solute has a vapor pressure of zero, so the vapor pressure
of the solution is less than the vapor pressure of the solvent.
Thus, a higher temperature is needed for the vapor pressure to reach
the surrounding pressure, and the boiling point is elevated.

Put in chemical potential terms, at the boiling point, the liquid phase and
the gas (or vapor) phase have the same chemical potential (or vapor
pressure) meaning that they are energetically equivalent.
The chemical potential is dependent on the temperature, and at other
temperatures either the liquid or the gas phase has a lower chemical
potential and is more energetically favorable than the other phase.
This means that when a nonvolatile solute is added, the chemical
potential of the solvent in the liquid phase is decreased by dilution, but
the chemical potential of the solvent in the gas phase is not affected.
This means in turn that the equilibrium between the liquid and gas phase
is established at another temperature for a solution than a pure liquid,
i.e., the boiling point is elevated.

The phenomenon of freezing-point depression is analogous to boiling point
elevation.
However, the magnitude of the freezing point depression is larger than
the boiling point elevation for the same solvent and the same
concentration of a solute.
Because of these two phenomena, the liquid range of a solvent is
increased in the presence of a solute.
The extent of boiling-point elevation can be calculated by
applying Clausius–Clapeyron relation and Raoult's law together with the
assumption of the non-volatility of the solute.
The result is that in dilute ideal solutions, the extent of boiling-point
elevation is directly proportional to the molal concentration (amount of
substance per mass) of the solution according to the equation:
ΔTb = Kb · bB
where the boiling point elevation, is defined as
Tb (solution) - Tb (pure solvent).

Kb, the ebullioscopic constant, which is dependent on the properties of the
solvent. It can be calculated as Kb = RTb
2M/ΔHv,
where R is the gas constant, and Tb is the boiling temperature of the pure
solvent [in K], M is the molar mass of the solvent, and ΔHv is the heat of
vaporization per mole of the solvent.
bB is the molality of the solution, calculated by taking dissociation into
account since the boiling point elevation is a colligative property, dependent
on the number of particles in solution.
This is most easily done by using the van 't Hoff factor i as bB = bsolute · i. The
factor i accounts for the number of individual particles (typically ions) formed
by a compound in solution.

6. Dewpointandwetbulbdepression
For warmer temperatures than 60, the cooling is between about 1/3
and 1/2 the dew-point depression.
If the temperature was 42 with a dew-point of 15 and it started
raining, the temperature and dew-point would wet-bulb out to a chilly
33 degrees Fahrenheit.
A quick technique that many forecasters use to determine the wet-bulb
temperature is called the "1/3 rule".
The technique is to first find the dew-point depression (temperature
minus dew-point).
Then take this number and divide by 3. Subtract this number from the
temperature. You now have an approximation for the wet-bulb
temperature.

Vapour pressure can be determined from the dew point of an air-water
mixture.
The temperature at which the dew point occurs is determined by
observing condensation on a smooth, cool surface such as a mirror.
This temperature can be related to vapour pressure using a
psychrometric chart.
The formation of dew is detected photoelectrically
Suppose the temperature is 42 degrees Fahrenheit with a dew-point of
15 degrees Fahrenheit.
The dew-point depression is 42-15 = 27. Now divide 27 by 3 = 9. Now
subtract 9 from the original temperature of 42. 42 - 9 = 33.
For temperatures between 30 and 60 degrees F, the 1/3 rule works
quite well.
For warmer temperatures than 60, the cooling is between about 1/3
and 1/2 the dew-point depression.

If the temperature was 42 with a dew-point of 15 and it started raining,
the temperature and dew-point would wet-bulb out to a chilly 33 degrees
Fahrenheit.
As dew-point depression or temperature increase, the evaporation
potentialincreases.
This technique does not give the exact wet bulb temperature but it does
give a pretty close approximation.
Warmer air will cool at a greater rate than colder air since more water
vapor can evaporate into warm air.
Evaporation is a cooling process that absorbs latent heat, therefore the
more evaporation the more cooling.

7. Suctionpotential,or by usingtheisopiesticmethod
The isopiestic method consists of equilibrating both a sample and a
reference material in an evacuated desiccator until equilibrium is reached
at 25°C.
The moisture content of the reference material is then determined and
the aw obtained from the sorption isotherm.
Since the sample was in equilibrium with the reference material, the aw
of both is the same.

8. Bithermalequilibrium(ThermocouplePsychrometer)
Water activity measurement is based on wet bulb temperature
depression.
A thermocouple is placed in the chamber where the sample is
equilibrated. Water is then sprayed over the thermocouple before it is
allowed to evaporate, causing a decrease in temperature.
The drop in temperature is related to the rate of water evaporation from
the surface of the thermocouple, which is a function of the relative
humidity in equilibrium with the sample.

9. Electrichygrometers
Most hygrometers are electrical wires coated with hygroscopic salts or
sulfonated polystyrene gel in which conductance or capacitance changes
as the coating absorbs moisture from the sample.
The major disadvantage of this type of hygrometer is the tendency of
the hygroscopic salt to become contaminated with polar compounds,
resulting in erroneous aw determinations.

10. Hairhygrometers
Hair hygrometers are based on the stretching of a fibre when exposed to
high water activity.
They are less sensitive than other instruments at lower levels of activity
(<0.03 aw) and the principal disadvantage of these types of meters is the
time delay in reaching equilibrium and the tendency to hysteresis.
Today we find many brands of water activity meters in the market. Most
of these meters are based on the relationship between ERH and the food
system, but differ in their internal components and configuration of
software used.
One of the water activity meters most used today is the AcquaLab
Series 3 Model TE, developed by Decagon Devices, which is based on the
chilled-mirror dew point method.

This instrument is a temperature controlled water activity meter that
allows placement of the sample in a temperature stable environment
without the use of an external water bath.
The temperature can be selected on the screen and is monitored and
controlled with thermoelectric components.
Most of the older generations of water activity instruments are based
on a temperature-controlled environment.
Therefore, a margin of error greater than 5% can be expected due to
temperature variations.
This equipment is highly recommended for measuring water activity in
fruits and vegetables since it measures a wide range of water activity.
The major advantages of the chilled-mirror dew point method are
accuracy, speed, ease of use and precision.
 The AquaLab's range is from 0.030 to 1.000aw, with a resolution of
±0.001aw and accuracy of ±0.003aw.

Measurement time is typically less than five minutes.
Capacitance sensors have the advantage of being inexpensive, but are
not usually as accurate or as fast as the chilled-mirror dew point
method.
Capacitive instruments measure over the entire water activity range 0 to
1.00 aw, with a resolution of ±0.005aw and accuracy of ±0.015aw.
Some commercial instruments can complete measurements in five minutes
while other electronic capacitive sensors usually require 30 to 90 minutes
to reach equilibrium relative humidity conditions.
The hair hygrometer uses the characteristic of the hair that its length
expands or shrinks response to the relative humidity. ...
The length of human hair from which liquid are removed increases by 2
to 2.5% when relative humidity changes by 0 to 100%.

Principleof Hairhygrometer
Due to humidity, several materials undergo a change in physical,
chemical and electrical properties.
This property is used in a transducer designed and calibrated to
directly read the relative humidity.
Certain hygroscopic materials, such as human hair, animal membranes,
wood, paper, etc., undergo changes in the linear dimensions when they
absorb moisture from the surrounding air.
This change in the linear dimension is used as the measurement of the
humidity present in the air.

Workingof hairhygrometer:
When air humidity is to be measured, this air is made to surround
the hair arrangement and the hair arrangement absorbs moisture from
the surrounding air and expands or contracts in the linear direction.
This expansion or contraction of the hair arrangement moves the arm
and the link and, therefore, the pointer to a suitable position on the
calibrated scale and, therefore, indicates the humidity present in the
air / atmosphere.
These Hair hydrometers are called membrane hydrometers when the
sensing element is a membrane.

Human hair is used as a humidity sensor.
The hair is arranged on a parallel beam and separated from each other to
expose them to the surrounding air / atmosphere.
Number of hairs are placed in parallel to increase the mechanical strength.
This hair arrangement is placed under a small tension by the use of a
tension spring to ensure proper functioning.
The hair arrangement is connected to an arm and a link arrangement and
the link is attached to a pointer rotated at one end. The pointer sweeps
over a calibrated scale of humidity.

Applicationof Hairhygrometer
These hydrometers are used in the temperature range of 0’C to 75’C.
These hydrometers are used in the range of relative humidity (relative
humidity) from 30 to 95%.
Limitations of the hydrometer for the hair
These hydrometers are slow in response
If the hair hydrometer is used constantly, its calibration tends to
change.

Advantages
The main advantages of this technology are its rather low cost
and a greater insensitivity to volatile substances such as alcohol
and propylene glycol.
Disadvantages
The main disadvantages of this technology are the loss of
efficiency of the sensor over time and fouling issues.
Advantages& Disadvantagesof WaterActivity

Water activity

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